1. Field of the Invention
Generally, the present disclosure relates to manufacturing processes, and, more particularly, to the scheduling of product streams in a manufacturing environment, such as a semiconductor facility, in which one or more different product types are processed by process and metrology tools based on scheduling regimes.
2. Description of the Related Art
Today's global market forces manufacturers of mass products to offer high quality products at a low price. It is thus important to improve yield and process efficiency to minimize production costs. This holds especially true in industrial fields, in which highly complex process tools operate on complex products according to specified process parameters that may vary between different product types. A prominent example in this respect represents the field of semiconductor fabrication, since, here, it is essential to combine cutting-edge technology with mass production techniques. It is, therefore, the goal of semiconductor manufacturers to reduce the consumption of raw materials and consumables while at the same time improve process tool utilization. The latter aspect is especially important since, in modern semiconductor facilities, equipment is required which is extremely cost-intensive and represents the dominant part of the total production costs.
Integrated circuits, as one example for a mass product, are typically manufactured in automated or semi-automated facilities, thereby passing through a large number of process and metrology steps to complete the device. The number and the type of process steps and metrology steps a product, such as a semiconductor device, has to go through depends on the specifics of the product to be fabricated. For example, a typical process flow for an integrated circuit may include a plurality of photolithography steps to image a circuit pattern for a specific device layer into a resist layer, which is subsequently patterned to form a resist mask for further processes for structuring the device layer under consideration by, for example, etch or implant processes, deposition processes, heat treatments, cleaning processes and the like. Thus, layer after layer, a plurality of process steps are performed based on a specific lithographic mask set for the various layers of the specified device. For instance, a sophisticated CPU requires several hundred process steps, each of which has to be carried out within specified process margins so as to fulfill the specifications for the device under consideration.
In many production plants, such as semiconductor facilities, usually a plurality of different product types are manufactured at the same time, such as memory chips of different design and storage capacity, CPUs of different design and operating speed and the like, wherein the number of different product types may even reach a hundred and more in production lines for manufacturing ASICs (application specific ICs). Since each of the different product types may require a specific process flow, specific settings in the various process tools, such as different mask sets for the lithography, different process parameters for deposition tools, etch tools, implantation tools, chemical mechanical polishing (CMP) tools, furnaces and the like, may be necessary. Consequently, a plurality of different tool parameter settings and product types may be encountered simultaneously in a manufacturing environment.
Hereinafter, the parameter setting for a specific process in a specified process tool or metrology or inspection tool may commonly be referred to as process recipe or simply as recipe. Thus, a large number of different process recipes, even for the same type of process tools, may be required which have to be applied to the process tools at the time the corresponding product types are to be processed in the respective tools. However, the sequence of process recipes performed in process and metrology tools or in functionally combined equipment groups as well as the recipes themselves may have to be frequently altered due to fast product changes and highly variable processes involved. As a consequence, the tool performance is a very critical manufacturing parameter as it significantly affects the overall production costs of the individual products, wherein one important aspect of the tool performance is throughput.
Therefore, in the field of semiconductor production, various strategies are practiced in an attempt to optimize the stream of products for achieving a high yield with moderate consumption of raw materials. In semiconductor plants, substrates are usually handled in groups, called lots, wherein, in a frequently encountered strategy, the dispatching of a sequence of lots for a given group of process tools, in which at least a part of the manufacturing process is to be performed, is determined on the basis of the current state of the lots and the tools, such that an efficient processing of the lots may be achieved. Efficiency in terms of lot processing may be understood as a processing regime, in which throughput of the tools under consideration is maintained at a high level while also taking into consideration product-specific criteria, such as short cycle time, meeting customers' demands and the like. Thus, a so-called dispatch list may be established for each process under consideration when demanded by an operator or an automated supervising system, which may describe the sequence of releasing the various lots designated for the process under consideration in an attempt to finally obtain efficient routing of the released lots through the process flow under consideration.
When passing through the process flow under consideration, the respective lots may be “distributed” along the process line according to their manufacturing stage and may be queued at respective process tools, waiting for the further processing in a dedicated process tool. The material currently processed in a tool or currently waiting for processing in the queue associated with the process tool under consideration is referred to as “work in progress” (WIP). During the specific process flow a lot has to undergo, the same process tool or tool group may have to be used repeatedly, although at different points in the process flow. For example, for many lithography processes, the same process tool(s) may be used to define different device layers, as previously described, so that a corresponding queue “in front” of a lithography module may comprise, in addition to substrate groups belonging to different product types, lots of the same product type at different manufacturing stages. Due to the complexity of the entire process flow, the product lots waiting for processing at a specific process tool, even when representing the same product type, may be present in very different amounts, which may not be correlated to the number of lots corresponding to a previous or subsequent manufacturing stage, although all lots of the product type under consideration have to pass through the dedicated process flow for this product type. For example, the flow of lots through the process line may be considered as a highway with heavy traffic, wherein, at different positions between an origin and the destination, very different traffic densities may occur, in particular when many sections of the distance between the origin and the destination have to passed several times. The conventional dispatch regimes take into consideration the situation at the respective points, i.e., process tools, in order to attempt to optimize the product stream through this process tool or process tool group, wherein the dispatch list may represent the priority for the processing of the various lots at this point according to the above-specified criteria. However, in addition to such important aspects, such as tool utilization, providing products on demand and the like, other criteria may have a strong influence on the overall performance of a manufacturing environment. For instance, the WIP in the facility may not be maintained at an arbitrarily high value, since this may result in reduced flexibility for responding to changes of factors, such as customer demands, global economic situation and the like. For example, in semiconductor facilities, the various processes may be highly interrelated in that the process result may depend on the queue time of a specific process so that a high amount of WIP at this process may result in significant losses, when the queue time may reach a non-tolerable high value, which may be referred to as queue time violation, due to a non-predictable event, such as tool failure and the like.
Hence, frequently, additional strategies may be applied in complex manufacturing environments that are aiming at controlling the WIP according to predefined criteria, such as maintaining the WIP constant (CONWIP), tightly controlling the WIP between pairs of adjacent operations and the like. These “supervising” control strategies may overrule the “local” control of the work stream at the individual process tools so as to control the WIP in the system. However, in the complex system of a semiconductor facility, a restriction of the WIP on the basis of conventional regimes as described above may be very difficult to implement due to the high number of different tool characteristics associated with process tools, such as furnaces, lithography tools, implanters, polish tools and the like, due to the high number of process steps, the variability of the availability of the process tools, the high investment costs and in particular the high number of re-entrant processes.
The present disclosure is directed to various methods and systems that may avoid, or at least reduce, the effects of one or more of the problems identified above.